1. Field of the Invention
The present invention relates generally to radio frequency communication systems. In particular although not exclusively the present invention relates to an apparatus for applying a load to a given point within a network for diagnostic purposes.
2. Discussion of the Background Art
Quality of Service (QOS) is of major importance to today's communication network providers. One of the major factors effecting QOS in most modern communication is interference. The two most appreciable forms of interference present in most communication systems result from Active and Passive intermodulation. In each case multiple transmitting frequencies combine in ways that cause interference to receiving equipment.
In the case of Active Intermodulation (AIM) interference the transmitter or receiver actively amplify interfering signals in the in the environment that cause harmful interference. Passive Intermodulation (PIM) interference is similar to active intermodulation interference except that it almost occurs exclusively in passive elements when two or more frequencies are simultaneously present. When signals F1 and F2 for example encounter a non-linear device they combine as follows, mF1±nF2, (m,n=1, 2, 3 . . . ) to produce interfering signals.
Presently it has been relatively difficult to test for PIM on-site. Historically the equipment required to perform the testing was rather large and cumbersome and not readily suited for in-field deployment and has been widely considered by most in the communications industry as being impractical. Typically such on-site PIM testing requires each junction, line and interconnect to be checked. Without a PIM tester on-site, this operation is extremely labour intensive, requiring a technician to physically check/remake each connection as installed, and as such is extremely costly.
To allow for on-site analysis of PIM interference along with other communication system parameters the applicant has devised a number of portable test units which are the subject of co-pending U.S. application Ser. No. 11/936,968 filed 8 Nov. 2007 and U.S. application Ser. No. 11/941,712 filed 10 Oct. 2007 the contents of which are herein incorporated by reference.
While the portable test apparatus developed by the applicant greatly reduce the time and cost involved indentifying sources of PIM interference in a communications system, a technician is still none the less required to attach a test load to a various points in order to obtain a reading for a given section of the network. Typically the test load are made from length of coiled electrical cable, such a load can be extremely bulky and unwieldy to use particularly in confined areas. In addition to this the use of different brands of cable to construct the desired load, means that the PIM tolerance for loads of similar resistance can vary greatly. This variance can affect the accuracy of the measurement of PIM interference within the system.
Accordingly the applicant has realised that there is a need for a standardised test load for the measurement of PIM interference within a communications system, which is relatively compact and easy to use.